1. Field of the Invention
The present invention relates to a radio-over fiber (RoF) system, and more particularly, to an electroabsorption duplexer including an optical amplifier, a photodetector, and an optical modulator which are monolithically integrated to be suitable to analog optical communication.
2. Description of the Related Art
In analog optical transmission, optical power is modulated in accordance with an electrical signal having a predetermined frequency so that the electrical signal is restored from an optical signal after being transmitted via optical fiber. Many studies on radio-over fiber (RoF) link optical transmission technology for converting a radio frequency (RF) signal including a modulated signal (e.g., a binary phase shift keying (BPSK) signal, a quadrature phase shift keying (QPSK) signal, or a quadrature amplitude modulation (QAM) signal) into an optical signal and transmitting the optical signal via optical fiber in the transmission procedure have been performed.
In the RoF link optical transmission technology, a ratio of an RF signal recovered by a photodetector to an RF signal input to an optical modulator is defined as an RF gain. Increasing the RF gain is essential to the RoF link optical transmission technology. Accordingly, an optical modulator having high performance of electro-optic conversion and a photodetector having high performance of opto-electric conversion are required.
In the RoF link optical transmission technology, when an RF increases, the amount of transmittable data also increases, but a transmission distance from an antenna to a wireless terminal decreases. As a result, a lot of base stations (BS) are needed. Accordingly, a structure in which many functions are concentrated on a central office (CO) and a BS is simplified, thereby decreasing the price of the BS, is preferable.
FIG. 1A is a block diagram of a conventional RoF link optical transmission system. Referring to FIG. 1A, in the conventional RoF link optical transmission system, a light source is in a CO 10. The CO 10 is connected to a BS 30 via an optical fiber 20. The BS 30 transmits a wireless signal to a wireless terminal (WT) 40 through a transmitting antenna 36 and a receiving antenna 42 and receives a wireless signal from the WT 40 through a transmitting antenna 44 and a receiving antenna 38.
FIG. 1B is a detailed block diagram of the BS 30 illustrated in FIG. 1A. In a RoF link, the BS 30 is usually implemented by a duplexer in which the transmitting antenna 36, the receiving antenna 38, a photodetector 32, and an optical modulator 34 are integrated.
Time-division transmission is suitable for transmission using the BS 30 illustrated in FIG. 1B. For example, during a particular time slot, light including an RF signal is transmitted from the CO 10 to the BS 30 and the RF signal is restored by the photodetector 32 in the BS 30 and is wirelessly transmitted to the WT 40. During another particular time slot, light from the CO 10 passes through the photodetector 32 as it is, is then modulated by the optical modulator 34, and is then transmitted to the CO 10. Here, a wireless signal transmitted from the WT 40 is used for the optical modulation. In the RoF link optical transmission technology, essential devices such as the photodetector 32 and the optical modulator 34 used in the BS 30 should be designed to be suitable for analog transmission and to be easily manufactured.
Generally, in an optical modulator, an RF gain is proportional to the square of the intensity of output light and to the square of a transfer function slope. However, when the transfer function slope of the optical modulator increases, light insertion loss also increases, and therefore, the intensity of output light decreases. Usually, the light insertion loss of a multi-quantum well electro-absorption (EA) modulator is about 10 dB and is increased when a transfer function slope is increased. Most of the insertion loss of 10 dB occurs because an optical mode in the optical modulator disagrees with an optical mode in an optical fiber. A method of integrating a spot size converter into an optical modulator and a method of integrating an optical amplifier are used to reduce light insertion loss.
FIG. 2 is a perspective view of a semiconductor laser diode into which a conventional spot size converter (SSC) is integrated. Referring to FIG. 2, the SSC is connected to a laser using a butt-joint coupling method. The thickness of an optical waveguide is uniform in the laser and gradually decreases when the optical waveguide passes a butt-joint coupling portion. The structure illustrated in FIG. 2 is obtained when a selective area growth (SAG) method, a highly functional epitaxial growth method, is used. In the structure, the thickness of the waveguide of the SSC gradually decreases in a direction in which light proceeds and eventually becomes 0.2 μm or less at the end of the waveguide.
In detail, a laser area includes an n-type electrode layer 11, an n-type indium phosphide (InP) clad layer 12, an n-type current blocking layer 13, a p-type current blocking layer 14, a p-type clad layer 15, and a p-type electrode layer 16. An SSC area includes a passive optical waveguide 17, which is connected to a laser active layer 19 included within the laser area via a butt-joint interface 18 and tapers away.
To manufacture the above-described SSC, the SAG method is used. However, when the SAG method is used, the composition of a substance is changed, causing stress to occur in a substance layer. As a result, semiconductor crystals may be deteriorated. To manufacture the SSC using the SAG method without the deterioration of semiconductor crystals, it is necessary to very strictly keep the growth conditions for semiconductor crystals. However, since conventional methods of manufacturing the SSC provides a very small tolerance for the growth conditions, there are many difficulties in growing high-quality semiconductor crystals.
FIG. 3 is a perspective view of a structure in which a conventional SSC, an optical amplifier, and an EA modulator are monolithically integrated. Referring to FIG. 3, a Fe-doped InP layer, i.e., a current blocking layer 22, an indium gallium arsenide phosphide (InGaAsP) passive optical waveguide 23, and an n-type InP spacer 24 are grown on an n-type InP substrate 21 using an SAG method. Next, etching is performed so that the passive optical waveguide 23 has a width of about 1 μm. Thereafter, the current blocking layer 22 is re-grown and planarized. Next, an active layer 25 for the optical amplifier and the EA modulator is grown using the SAG method. Thereafter, the active layer 25 in the optical amplifier is etched so that the width thereof is reduced in a side direction. Next, a p-type InP clad layer 26, a p-type InP cap layer, and an InGaAs contact layer are sequentially grown.
The structure illustrated in FIG. 3 has a dual-optical waveguide including a lower optical waveguide 23 and an upper optical waveguide 25. The lower optical waveguide 23 functions to increase an optical coupling with an optical fiber and the upper optical waveguide 25 functions to allow an optical mode to easily move from the optical amplifier to the lower optical waveguide 23. In detail, the lower optical waveguide 23 is grown using the SAG method such that the thickness thereof gradually decreases in a direction in which the optical mode propagates. Accordingly, the optical mode gradually increases and the efficiency of optical coupling with an optical fiber also increases. The upper optical waveguide 25 is also grown using the SAG method such that a width decreases in the direction in which the optical mode propagates, thereby allowing the optical mode to easily move to the lower optical waveguide 23.
The optical mode is strongly restrained in the optical modulator and the optical amplifier, thereby providing huge optical modulation or amplification efficiency (while the efficiency of optical coupling between an optical fiber and the strongly-restrained optical mode is very low). In the SSC, the optical mode is weakly restrained, that is, the size of the optical mode increases, and therefore, the efficiency of optical coupling with an optical fiber increases. As a result, all of the optical modulation efficiency, the optical amplification efficiency, and the optical coupling efficiency are maximized. However, since the structure illustrated in FIG. 3 is manufactured using the SAG method, manufacturing processes are complicated and it is difficult to manufacture high-quality devices.
When an output optical current increases, a photodetector provides a more advantageous RF gain. In other words, the photodetector is more advantageous when it has a large responsivity and receives a lot of light. To increase the responsivity of the photodetector, it is important to decrease light loss occurring during optical coupling with an optical fiber.